This invention relates to a laser radar device that operates on an elapsed time basis and has a pulsating laser for emitting successive light pulses into a monitored region, a light receiving arrangement for receiving light impulses reflected by an object in the monitored region, and an evaluation unit that generates a distance signal indicative of a distance between the object and the laser radar device.
Such laser radar devices are known from German patent document DE 43 40 756 A1. The laser radar device disclosed therein has a pulsed laser which directs successive light pulses into a monitoring region. A light receiving arrangement detects light pulses that are reflected by an object located in the monitoring region and sends corresponding electrical signals to an evaluation unit. The evaluation unit generates a distance signal that is indicative of a distance between the object and the laser radar device on the basis of the speed of light and an elapsed time between the emission and the receipt of the light pulses. A light diverter is arranged between the pulsed laser and the monitoring region and continuously changes the orientation of the light pulses through the monitored region so that the entire monitored region will be illuminated.
Laser radar devices of this type have a variety of uses. They are particularly useful for detecting objects within a danger zone. In view of the ability of such laser radar devices to determine distances, it is possible to both detect the presence of an object and to provide distance information, which, when combined with the measured rotational angle of the light diverter, allows one to determine the precise position of the object.
The word “light” as used herein is not limited to visible light. The word “light” is intended to generally cover electromagnetic beams, such as UV-light, IR-light, as well as visible light, which are typically used in optoelectronic sensors.
A disadvantage of the known state of the art is encountered when such laser radar devices monitor large monitoring regions to detect objects with widely differing reflection characteristics. Such prior art laser radar devices therefore normally have a permanent, exceedingly high signal dynamic in the light receiving arrangement, including the subsequent signal processing. Such a high signal dynamic leads to relatively high production costs. It can also lead to a malfunctioning of the laser radar device. Reasons for such malfunctioning can for example be external interfering light sources which are in the immediate vicinity of the monitoring region so that stray light from them can affect the light receiving arrangement of the laser radar device. This can significantly interfere with the light receiving arrangement, the detection sensitivity of which must be set for the lowest expected signals received from dark objects in the monitoring region, because such interfering light sources generate significant amounts of stray light or background noise that becomes superimposed onto the measurement signal. The same problem is encountered when light pulses emitted by the laser radar device are reflected by an object that has a high reflectivity and is located in the immediate vicinity of the laser radar device. The high energy received signals cause saturation effects or noise interference which can significantly degrade the accuracy of the distance measurement. The prior art tried to overcome these problems for optoelectronic sensors by suppressing the interference and, for example, taking multiple measurements from which average values are derived. It is also known to repeat inaccurate measurements by changing the detection sensitivity of the light receiving arrangement. However, this approach is not usable with laser radar devices that change direction of the emitted light because successive light pulses involve changes in the beam directions so that different objects or object portions which are unrelated to each other would provide incompatible outputs.
This could be reduced by correspondingly reducing the light deflections. However, this is not feasible because the responsiveness of the laser radar device would correspondingly decrease.